Internal combustion (IC) engines can operate in a spark ignition (SI) mode, in which a nearly homogeneous air and fuel charge is spark-ignited within a combustion chamber. IC engines may also operate in a compression ignition mode, in which compression of a non-homogeneous air and fuel charge within a combustion chamber ignites the charge. Homogeneous charge compression ignition (HCCI) is a type of compression ignition in which air and fuel are thoroughly mixed in an engine cylinder before compression-initiated self-ignition. Worldwide regulatory initiatives to lower greenhouse gases, vehicular nitrogen oxides (NOx) and particulate matter (PM) emission levels have heightened interest in HCCI, as HCCI can combine the low-NOx exhaust emissions of gasoline engines with three-way catalysts with the high thermal efficiency associated with diesel engines.
In HCCI, enhanced air-fuel mixing occurs generally through direct fuel injection at an earlier stage than diesel fuel injection. Unlike conventional diesel combustion, HCCI combustion results from spontaneous auto-ignition at multiple points throughout the volume of charge gas. HCCI combustion typically occurs in two stages. A low temperature heat release (LTHR) occurs first, followed by a high temperature heat release (HTHR). LTHR50 is the time at the mid-point of LTHR and HTHR50 is the time at the mid-point of HTHR.
Broadening the main heat release event over more crank angles and reducing the maximum rate of pressure rise increases the operating range of a HCCI engine. Yao, et al., “An investigation on the effects of fuel chemistry and engine operating conditions on HCCI engine”, SAE Technical Paper Series 2008-01-1660; Lu, et al., “Experimental study and chemical analysis of n-heptane homogeneous charge compression ignition combustion with port injection of reaction inhibitors”, Combustion and Flame 149 (2007) 261-270.
While these attributes of HCCI are known, it has still proven difficult to operate HCCI engines over a wide range of loads for a number of reasons.
Since HCCI engines rely on auto-ignition, combustion phasing (the timing of auto-ignition) is inherently difficult to control. Combustion occurs very rapidly in HCCI engines and the high rate of pressure increase limits the higher load range of HCCI engines due to mechanical and noise concerns. Low load operation in HCCI mode can also be problematic, as some fuels, especially gasolines, do not readily auto-ignite at low loads. HCCI is also sensitive to fuel composition, Shibata, et al., “Correlation of Low Temperature Heat Release with Fuel Composition and HCCI Engine Combustion”, SAE Technical Paper Series 2005-01-0138.
Although external exhaust gas recirculation (EGR) and variable valve timing (VVT) help to control the combustion heat release characteristics and rate of pressure rise of HCCI and other IC engines, each of these design options has its detriments.
External EGR leads to a slow response rate since EGR gases must flow through the exhaust and EGR system. External EGR also requires substantial heat dissipation; EGR must often be cooled prior to introduction into the engine. Further, to achieve high load performance with EGR, a larger engine size is needed (due to the displacement of air by EGR), which leads to a loss of efficiency and power. While internal EGR strategies using VVT have faster response rates, some of these valve strategies such as delayed intake valve closure time, lead to decreases in power and efficiency.
Negative Valve Overlap (NVO) attempts to solve HCCI's low load auto-ignition problem by using early exhaust valve closing to trap burnt gases. The trapped gases assist with auto-ignition during a subsequent compression stroke. In another approach called re-breathing, the exhaust valve reopens during the intake stroke to allow burnt gases to reenter the cylinder from the exhaust port. Multiple fuel injection has also been used in an effort to optimize fuel composition across a range of load conditions.
Notwithstanding the aforementioned efforts to optimize HCCI engine combustion phasing, the need continues to exist for practical fuels and methods that will enable IC engines to operate in advanced combustion modes such as the HCCI mode over a broader operating range, with fast response to load and mode changes, while retaining fuel economy and emissions benefits. Ideally, such fuels and methods would operate effectively at high and low engine loads, would achieve improved peak NOx and PM emission levels, and would enhance thermal efficiency without the mechanical complexities and thermal inefficiencies associated with known engine designs.